some parts of the United States, and the now common occurrence of some
of them in districts where they were unknown forty years ago, are
startling facts, and it is a very interesting question how far man's
acts or neglects may have occasioned the change. See Third Anual Report
of Massachusetts State Board of Health for 1873. The causes and remedies
of the insalubrity of Rome and its environs have been for some time the
object of careful investigation, and many valuable reports have been
published on the subject. Among the most recent of these are: Relazione
sulle condizioni agrarie ed igieniche della Campagna di Roma, per
Raffaele Pareto; Cenni Storici sulla questione dell' Agro Romano di G.
Guerzoni; Cenni sulle condizioni Fisico-economiche di Roma per F.
Giordano; and a very important paper in the journal Lo Sperimentale for
1870, by Dr. D. Pantaleoni.
There are climates, parts of California, for instance, where the flesh
of dead animals, freely exposed, shows no tendency to putrefaction but
dries up and may be almost indefinitely preserved in this condition. Is
this owing to the absence of destructive animalcular life in such
localities, and has man any agency in the introduction and
naturalization of these organisms in regions previously not infested by
them ]
CHAPTER III.
THE WOODS.
The habitable earth originally wooded--General meteorological influence
of the forest--Electrical action of trees--Chemical influence of
woods--Trees as protection against malaria--Trees as shelter to ground
to the leeward--Influence of the forest as inorganic on
temperature--Thermometrical action of trees as organic--Total influence
of the forest on temperature--Influence of forests as inorganic on
humidity of air and earth--Influence as organic--Balance of conflicting
influences--Influence of woods on precipitation--Total climatic action
of the forest--Influence of the forest on humidity of soil--The forest
in winter--Summer rain, importance of--Influence of the forest on the
flow of springs--Influence of the forest on inundations and
torrents--Destructive action of torrents--Floods of the
Ardeche--Excavation by torrents--Extinction of torrents--Crushing force
of torrents--Transporting power of water--The Po and its
deposits--Mountain slides--Forest as protection against
avalanches--Minor uses of the forest--Small forest plants and vitality
of seeds--Locusts do not breed in forests--General functions of
forest--General consequences of destruction of--Due proportion of
woodland--Proportion of woodland in European countries--Forests of Great
Britain--Forests of France--Forests of Italy--Forests of
Germany--Forests of United States--American forest trees--European and
American forest trees compared--The forest does not furnish food for
man--First removal of the forest--Principal causes of destruction of
forest--Destruction and protection of forests by governments--Royal
forests and game-laws--Effects of the French revolution--Increased
demand for lumber--Effects of burning forest--Floating of
timber--Restoration of the forest--Economy of the forest--Forest
legislation--Plantation of forests in America--Financial results of
forest plantations--Instability of American life.
The Habitable Earth originally Wooded.
There is good reason to believe that the surface of the habitable earth,
in all the climates and regions which have been the abodes of dense and
civilized populations, was, with few exceptions, already covered with a
forest growth when it first became the home of man. This we infer from
the extensive vegetable remains--trunks, branches, roots, fruits, seeds,
and leaves of trees--so often found in conjunction with works of
primitive art, in the boggy soil of districts where no forests appear to
have existed within the eras through which written annals reach; from
ancient historical records, which prove that large provinces, where the
earth has long been wholly bare of trees, were clothed with vast and
almost unbroken woods when first made known to Greek and Roman
civilization; [Footnote: The recorded evidence in support of the
proposition in the text has been collected by L. F. Alfred Maury, in his
Histoire des grandes Forets de la Gauls et de l'ancienne France, and by
Becquerel, in his important work, Des climats et de l'Influence
qu'exercent les Sols boises et non boises, livre ii., chap. i. to iv.
We may rank among historical evidences on this point, if not technically
among historical records, old geographical names and terminations
etymologically indicating forest or grove, which are so common in many
parts of the Eastern Continent now entirely stripped of woods--such as,
in Southern Europe, Breuil, Broglio, Brolio, Brolo; in Northern, Bruhl,
and the endings -dean, -den, -don, -ham, -holt, -horst, -hurst, -lund,
-shaw, -shot, -skog, -skov, -wald, -weald, -wold, -wood.] and from the
state of much of North and of South America, as well as of many islands,
when they were discovered and colonized by the European race. [Footnote:
The island of Madeira, whose noble forests wore devastated by fire not
Iong after its colonization by European settlors, takes its name from
the Portuguese word tor wood.]
These evidences are strengthened by observation of the natural economy
of our time; for, whenever a tract of country once inhabited and
cultivated by man, is abandoned by him and by domestic animals, and
surrendered to the undisturbed influences of spontaneous nature, its
soil sooner of later clothes itself with herbaceous and arborescent
plants, and, at no long interval, with a dense forest growth. Indeed,
upon surfaces of a certain stability and not absolutely precipitous
inclination the special conditions required for the spontaneous
propagation of trees may all be negatively expressed and reduced to
these three: exemption from defect or excess of moisture, from perpetual
frost, and from the depredations of man and browsing quadrupeds. Where
these requisites are secured, the hardest rock is as certain to be
overgrown with wood as the most fertile plain, though, for obvious
reasons, the process is slower in the former than in the latter case.
Lichens and mosses first prepare the way for a more highly organized
vegetation. They retain the moisture of rains and dews, and bring it to
act, in combination with the gases evolved by their organic processes,
in decomposing the surface of the rocks they cover; they arrest and
confine the dust which the wind scatters over them, and their final
decay adds new material to the soil already half formed beneath and upon
them. A very thin stratum of mould is sufficient for the germination of
seeds of the hardy evergreens and birches, the roots of which are often
found in immediate contact with the rock, supplying their trees with
nourishment from a soil deepened and enriched by the decomposition of
their own foliage, or sending out long rootlets into the surrounding
earth in search of juices to feed them.
The eruptive matter of volcanoes, forbidding as is its aspect, does not
refuse nutriment to the woods. The refractory lava of Etna, it is true,
remains long barren, and that of the great eruption of 1669 is still
almost wholly devoid of vegetation.
[Footnote: Even the volcanic dust of Etna remains very long
unproductive. Near Nicolosi is a great extent of coarse black sand,
thrown out in 1669, which, for almost two centuries, lay entirely bare,
and can be made to grow plants only by artificial mixtures and much
labor.
The increase in the price of wines, in consequence of the diminution of
the product from the grape disease, however, has brought even these
ashes under cultivation. "I found," says Waltershausen, referring to the
years 1861-62, "plains of volcanic sand and half-subdued lava streams,
which twenty years ago lay utterly waste, now covered with fine
vineyards. The ashfield of ten square miles above Nicolosi, created by
the eruption of 1669, which was entirely barren in 1835, is now planted
with vines almost to the summits of Monte Rosso, at a height of three
thousand feet" Ueber den Sicilianischen Ackerbau, p. 19.] But the cactus
is making inroads even here, while the volcanic sand and molten rock
thrown out by Vesuvius soon become productive. Before the great eruption
of 1631 even the interior of the crater was covered with vegetation.
George Sandys, who visited Vesuvius in 1611, after it had reposed for
several centuries, found the throat of the volcano at the bottom of the
crater "almost choked with broken rocks and trees that are falne
therein." "Next to this," he continues, "the matter thrown up is ruddy,
light, and soft: more removed, blacke and ponderous: the uttermost brow,
that declineth like the seates in a theater, flourishing with trees and
excellent pasturage. The midst of the hill is shaded with chestnut
trees, and others bearing sundry fruits." [Footnote: A Relation of a
Journey Begun An. Dom. 1610, lib. 4, p. 260, edition of 1615. The
testimony of Sandys on this point is confirmed by that of Pighio,
Braccini, Magliocco, Salimbeni, and Nicola di Rubco, all cited by Roth,
Der Vesuv., p. 9. There is some uncertainty about the date of the last
eruption previous to the great one of 163l. Ashes, though not lava,
appear to have been thrown out about the year 1500, and some chroniclers
have recorded an eruption in the year 1306; but this seems to be an
error for 1036, when a great quantity of lava was ejected. In 1130,
ashes were thrown out for many days. I take these dates from the work of
Roth just cited.] I am convinced that forests would soon cover many
parts of the Arabian and African deserts, if man and domestic animals,
especially the goat and the camel, were banished from them. The hard
palate and tongue and strong teeth and jaws of this latter quadruped
enable him to break off and masticate tough and thorny branches as large
as the finger. He is particularly fond of the smaller twigs, leaves, and
seed-pods of the sont and other acacias, which, like the American
Robinia, thrive well on dry and sandy soils, and he spares no tree the
branches of which are within his reach, except, if I remember right, the
tamarisk that produces manna. Young trees sprout plentifully around the
springs and along the winter water-courses of the desert, and these are
just the halting stations of the caravans and their routes of travel. In
the shade of these trees, annual grasses and perennial shrubs shoot up,
but are mown down by the hungry cattle of the Bedouin, as fast as they
grow. A few years of undisturbed vegetation would suffice to cover such
points with groves, and these would gradually extend themselves over
soils where now scarcely any green thing but the bitter colocynth and
the poisonous foxglove is ever seen.
General Meteorological Influence of the Forest.
The physico-geographical influence of forests may be divided into two
great classes, each having an important influence on vegetable and on
animal life in all their manifestations, as well as on every branch of
rural economy and productive industry, and, therefore, on all the
material interests of man. The first respects the meteorology of the
countries exposed to the action of these influences; the second, their
superficial geography, or, in other words, the configuration,
consistence, and clothing of their surface.
For reasons assigned in the first chapter, and for others that will
appear hereafter, the meteorological or climatic branch of the subject
is the most obscure, and the conclusions of physicists respecting it
are, in a great degree, inferential only, not founded on experiment or
direct observation. They are, as might be expected, somewhat discordant,
though one general result is almost universally accepted, and seems
indeed too well supported to admit of serious question, and it may be
considered as established that forests tend to mitigate, at least within
their own precincts, extremes of temperature, humidity, and drought. By
what precise agencies the meteorological effects of the forest are
produced we cannot say, because elements of totally unknown value enter
into its action, and because the relative intensity of better understood
causes cannot be measured or compared. I shall not occupy much space in
discussing questions which at present admit of no solution, but I
propose to notice all the known forces whose concurrent or conflicting
energies contribute to the general result, and to point out, in some
detail, the value of those influeuces whose mode of action has been
ascertained. Electrical Influence of Trees. The properties of trees,
singly and in groups, as exciters or conductors of electricity, and
their consequent influence upon the electrical state of the atmosphere,
do not appear to have been much investigated; and the conditions of the
forest itself are so variable and so complicated, that the solution of
any general problem respecting its electrical influence would be a
matter of extreme difficulty. It is, indeed, impossible to suppose that
a dense cloud, a sea of vapor, can pass over miles of surface bristling
with good conductors, without undergoing and producing some change of
electrical condition. Hypothetical cases may be put in which the
character of the change could be deduced from the known laws of
electrical action. But in actual nature, the elements are too numerous
for us to seize. The true electrical condition of neither cloud nor
forest could be known, and it could seldom be predicted whether the
vapors would be dissolved as they floated over the wood, or discharged
upon it in a deluge of rain. With regard to possible electrical
influences of the forest, wider still in their range of action, the
uncertainty is even greater. The data which alone could lead to
positive, or even probable, conclusions are wanting, and we should,
therefore, only embarrass our argument by any attempt to discuss this
meteorological element, important as it may be, in its relations of
cause and effect to more familiar and bettor understood meteoric
phenomena. It may, however, be observed that hail-storms--which were
once generally supposed, and are still held by many, to be produced by a
specific electrical action, and which, at least, appear to be always
accompanied by electrical disturbances--are believed, in all countries
particularly exposed to that scourge, to have become more frequent and
destructive in proportion as the forests have been cleared. Caimi
observes: "When the chains of the Alps and the Apennines had not yet
been stripped of their magnificent crown of woods, the May hail, which
now desolates the fertile plains of Lombardy, was much less frequent;
but since the general prostration of the forest, these tempests are
laying waste even the mountain-soils whose older inhabitants scarcely
knew this plague. [Footnote: There are, in Northern Italy and in
Switzerland, joint-stock companies which insure against damage by hail,
as well as by fire and lightning. Between the years 1854 and 1861, a
single one of these companies, La Riunione Adriatica, paid, for damage
by hail in Piedmont, Venetian Lombardy, and the Duchy of Parma, above
6,500,000 francs, or nearly $200,000 per year.] The paragrandini,
[Footnote: The paragrandine, or, as it is called in French, the
paragrele, is a species of conductor by which it has been hoped to
protect the harvests in countries particularly exposed to damage by
hail. It was at first proposed to employ for this purpose poles
supporting sheaves of straw connected with the ground by the same
material; but the experiment was afterwards tried in Lombardy on a large
scale, with more perfect electrical conductors, consisting of poles
secured to the top of tall trees and provided with a pointed wire
entering the ground and reaching above the top of the pole. It was at
first thought that this apparatus, erected at numerous points over an
extent of several miles, was of some service as a protection against
hail, but this opinion was soon disputed, and does not appear to be
supported by well-ascertained facts. The question of a repetition of the
experiment over a wide area has been again agitated within a very few
years in Lombardy; but the doubts expressed by very able physicists as
to its efficacy, and as to the point whether hail is an electrical
phenomenon, have discouraged its advocates from attempting it.] which
the learned curate of Rivolta advised to erect, with sheaves of straw
set up vertically, over a great extent of cultivated country, are but a
Liliputian imago of the vast paragrandini, pines, larches, and fire,
which nature had planted by millions on the crests and ridges of the
Alps and the Apennines." [Footnote: Cenni sulla Importansa e Coltura dei
Boschi, p. 6.] "Electrical action being diminished," says Meguscher,
"and the rapid congelation of vapors by the abstraction of heat being
impeded by the influence of the woods, it is rare that hail or
waterspouts are produced within the precincts of a large forest when it
is assailed by the tempest." [Footnote: Memoria sui Boschi, etc., p.
44.] Arthur Young was told that since the forests which covered the
mountains between the Riviera and the county of Montferrat had
disappeared, hail had become more destructive in the district of Acqui,
[Footnote: Travels in Italy, chap. iii.] and a similar increase in the
frequency and violence of hail-storms in the neighborhood of Saluzzo and
Mondovi, the lower part of the Valtelline, and the territory of Verona
and Vicenza, is probably to be ascribed to a similar cause. [Footnote:
Le Alpi che cingono l'Italia, i., p. 377. See "On the Influence of the
Forest in Preventing Hail-storms," a paper by Becquerel, in the Memoires
de l'Academie des Sciences, vol. xxxv. The conclusion of this eminent
physicist is, that woods do excercise, both within their own limits and
in their vicinity, the influence popularly ascribed to them in this
respect, and that the effect is probably produced partly by mechanical
and partly by electrical action.] Chemical Influence of the Forest. We
know that the air in a close apartment is appreciably affected through
the inspiration and expiration of gases by plants growing in it. The
same operations are performed on a gigantic scale by the forest, and it
has even been supposed that the absorption of carbon, by the rank
vegetation of earlier geological periods, occasioned a permanent change
in the constitution of the terrestrial atmosphere. [Footnote: "Long
before the appearance of man, ... they [the forests] had robbed the
atmosphere of the enormous quantity of carbonic acid it contained, and
thereby transformed it into respirable air. Trees heaped upon trees had
already filled up the ponds and marshes, and buried with them in the
bowels of the earth--to restore it to us, after thousands of ages, in
the form of bituminous coal and of anthracite--the carbon which was
destined to become, by this wonderful condensation, a precious store of
future wealth."--Clave, Etudes sur l'Economie Forestiere, p. 13.
This opinion of the modification of the atmosphere by vegetation is
contested.
Mossman ascribes the great luxuriance and special character of the
Australian and New Zealand forests, as well as other peculiarities of
the vegetation of the Southern hemisphere, to a supposed larger
proportion of carbon in the atmosphere of that hemisphere, though the
fact of such excess does not appear to have been established by chemical
analysis. Mossman, Origin of the Seasons. Edinburgh, 1869. Chaps. xvi.
and xvil.] To the effects thus produced are to be added those of the
ultimate gaseous decomposition of the vast vegetable mass annually shed
by trees, and of their trunks and branches when they fall a prey to
time. But the quantity of gases thus abstracted from and restored to the
atmosphere is inconsiderable--infinitesimal, one might almost say--in
comparison with the ocean of air from which they are drawn and to which
they return; and though the exhalations from bogs, and other low grounds
covered with decaying vegetable matter, are highly deleterious to human
health, yet, in general, the air of the forest is hardly chemically
distinguishable from that of the sand plains, and we can as little trace
the influence of the woods in the analysis of the atmosphere, as we can
prove that the mineral ingredients of landsprings sensibly affect the
chemistry of the sea. I may, then, properly dismiss the chemical, as I
have done the electrical, influences of the forest, and treat them both
alike, if not as unimportant agencies, at least as quantities of unknown
value in our meteorological equation. [Footnote: Schacht ascribes to the
forest a specific, if not a measurable, influence upon the constitution
of the atmosphere. "Plants imbibe from the air carbonic acid and other
gaseous or volatile products exhaled by animals or developed by the
natural phenomena of decomposition. On the other hand, the vegetable
pours into the atmosphere oxygen, which is taken up by animals and
appropriated by them. The tree, by means of its leaves and its young
herbaceous twigs, presents a considerable surface for absorption and
evaporation; it abstracts the carbon of carbonic acid, and solidifies it
in wood, fecula, and a multitude of other compounds. The result is that
a forest withdraws from the air, by its great absorbent surface, much
more gas than meadows or cultivated fields, and exhales proportionally a
considerably greater quantity of oxygen. The influence of the forests on
the chemical composition of the atmosphere is, in a word, of the highest
importance."--Les Arbres, p. 111.
See on this subject a paper by J. Jamin, in the Revue des Deux Mondes
for Sept. 15, 1864; and, on the effects of human industry on the
atmosphere, an article in Aus der Natur, vol. 29, 1864, pp. 443, 449,
465, et seq. See also Alfred Maury, Les Forete de la Gaule, p. 107.] Our
inquiries upon this branch of the subject will accordingly be limited to
the thermometrical and hygrometrical influences of the woods. There is,
however, a special protective function of the forest, perhaps, in part,
of a chemical nature, which may be noticed here.
Trees as a Protection against Malaria.
The influence of forests in preventing the diffusion of miasmatic vapors
is not a matter of familiar observation, and perhaps it does not come
strictly within the sphere of the present inquiry, but its importance
will justify me in devoting some space to the subject. "It has been
observed" (I quote from Becquerel) "that humid air, charged with
miasmata, is deprived of them in passing through the forest. Rigaud de
Lille observed localities in Italy where the interposition of a screen
of trees preserved everything beyond it, while the unprotected grounds
were subject to fevers." [Footnote: Becquerel, Des Climats, etc., p. 9.]
Few European countries present better opportunities for observation on
this point than Italy, because in that kingdom the localities exposed to
miasmatic exhalations are numerous, and belts of trees, if not forests,
are of so frequent occurrence that their efficacy in this respect can be
easily tested. The belief that rows of trees afford an important
protection against malarious influences is very general among Italians
best qualified by intelligence and professional experience to judge upon
the subject. The commissioners, appointed to report on the measures to
be adopted for the improvement of the Tuscan Maremme, advised the
planting of three or four rows of poplars, Populus alla, in such
directions as to obstruct the currents of air from malarious localities,
and thus intercept a great proportion of the pernicious
exhalations." [Footnote: Salvagnoli, Rapporto sul Bonificamento delle
Maremme Toscane, pp. xii., 124.] Maury believed that a few rows of
sunflowers, planted between the Washington Observatory and the marshy
banks of the Potomac, had saved the inmates of that establishment from
the intermittent fevers to which they had been formerly liable. Maury's
experiments have been repeated in Italy. Large plantations of sunflowers
have been made upon the alluvial deposits of the Oglio, above its
entrance into the Lake of Iseo, near Pisogne, and it is said with
favorable results to the health of the neighborhood. [Footnote: Il
Politecnico, Milano, Aprile e Maggio, 1863, p. 35.] In fact, the
generally beneficial effects of a forest wall or other vegetable screen,
as a protection against noxious exhalations from marshes or other
sources of disease, situated to the windward of them, are very commonly
admitted.
It is argued that, in these cases, the foliage of trees and of other
vegetables exercises a chemical as well as a mechanical effect upon the
atmosphere, and some, who allow that forests may intercept the
circulation of the miasmatic effluvia of swampy soils, or even render
them harmless by decomposing them, contend, nevertheless, that they are
themselves active causes of the production of malaria. The subject has
been a good deal discussed in Italy, and there is some reason to think
that under special circumstances the influence of the forest in this
respect may be prejudicial rather than salutary, though this does not
appear to be generally the case. [Footnote: Salvagnoli, Memorie sulle
Maremme Toscane, pp. 213, 214. The sanitary action of the forest has
been lately matter of much attention in Italy. See Rendiconti del
Congresso Medico del 1869 a Firenze, and especially the important
observations of Selmi, Il Miasma Palustre, Padua, 1870, pp. 100 et seq.
This action is held by this able writer to be almost wholly chemical,
and he earnestly recommends the plantation of groves, at least of belts
of trees, as an effectual protection against the miasmatic influence of
marshes. Very interesting observations on this point will be found in
Ebermayer, Die Physikalischen Einwirkungen des Waldes, Aschaffenburg,
1873, B. I., pp. 237 et seq., where great importance is ascribed to the
development of ozone by the chemical action of the forest. The
beneficial influence of the ozone of the forest atmosphere on the human
system is, however, questioned by some observers. See also the able
memoir: Del Miasma vegetale e delle Malattis Miasmatiche of Dr D.
Pantaleoni in Lo Sperimentale, vol. xxii., 1870.
The necessity of such hygienic improvements as shall render the new
capital of Italy a salubrious residence gives great present importance
to this question, and it is much to be hoped that the Agro Romano, as
well as more distant parts of the Campagna, will soon be dotted with
groves and traversed by files of rapidly growing trees. Many forest
trees grow with great luxuriance in Italy, and a moderate expense in
plantation would in a very few years determine whether any amelioration
of the sanitary condition of Rome can be expected from this measure.
It is said by recent writers that in India the villages of the natives
and the encampments of European troops, situated in the midst or in the
neighborhood of groves and of forests, are exempt from cholera. Similar
observations were also made in 18S4 in Germany when this terrible
disease was raging there. It is hence inferred that forests prevent the
spreading of this malady, or rather the development of those unknown
influences of which cholera is the result. These influences, if we may
believe certain able writers on medical subjects, are telluric rather
than meteoric; and they regard it as probable that the uniform moisture
of soil in forests may be the immediate cause of the immunity enjoyed by
such localities. See an article by Pettenkofer in the Sud-Deutsche
Presse, August, 1869; and the observations of Ebermayer in the work
above quoted, pp. 246 et seq.
In Australia and New Zealand, as well as generally in the Southern
Hemisphere, the indigenous trees are all evergreens, and even deciduous
trees introduced from the other side of the equator become evergreen. In
those regions, even in the most swampy localities, malarious diseases
are nearly, if not altogether, unknown. Is this most important fact due
to the persistence of the foliage Mossman, Origin of Climates, pp. 374,
393, 410, 425, et seq.] It is, at all events, well known that the great
swamps of Virginia and the Carolinas, in climates nearly similar to that
of Italy, are healthy even to the white man, so long as the forests in
and around them remain, but become very insalubrious when the woods are
felled. [Footnote: Except in the seething marshes of northern tropical
and subtropical regions, where vegetable decay is extremely rapid, the
uniformity of temperature and of atmospheric humidity renders all
forests eminently healthful. See Hohensten's observations on this
subject, Der Wald, p. 41; also A. Maury, Les Forets de la Gaule, p. 7.
The flat and marshy district of the Sologne in France was salubrious
until its woods were felled. It then became pestilential, but within the
last few years its healthfulness has been restored by forest
plantations. Jules Clave in Revue des Deux Mondes for 1st March, 1866,
p. 209. There is no question that open squares and parks conduce to the
salubrity of cities, and many observers are of opinion that the trees
and other vegetables with which such grounds are planted contribute
essentially to their beneficial influence. See an article in Aus der
Natur, xxii, p. 813.]
Trees as Shelter to Ground to the Leeward.
As a mechanical obstruction, trees impede the passage of air-currents
over the ground, which, as is well known, is one of the most efficient
agents in promoting evaporation and the refrigeration resulting from
it. [Footnote: It is perhaps too much to say that the influence of trees
upon the wind is strictly limited to the mechanical resistance of their
trunks, branches, and foliage. So far as the forest, by dead or by
living action, raises or lowers the temperature of the air within it, so
far it creates upward or downward currents in the atmosphere above it,
and, consequently, a flow of air towards or from itself. These
air-streams have a certain, though doubtless a very small, influence on
the force and direction of greater atmospheric movements.] In the
forest, the air is almost quiescent, and moves only as local changes of
temperature affect the specific gravity of its particles. Hence there is
often a dead calm in the woods when a furious blast is raging in the
open country at a few yards' distance. The denser the forest--as, for
example, where it consists of spike-leaved trees, or is thickly
intermixed with them--the more obvious is its effect, and no one can
have passed from the field to the wood in cold, windy weather, without
having remarked it. [Footnote: As a familiar illustration of the
influence of the forest in checking the movement of winds, I may mention
the well-known fact, that the sensible cold is never extreme in thick
woods, where the motion of the air is little felt. The lumbermen in
Canada and the Northern United States labor in the woods, without
inconvenience, when the mercury stands many degrees below the zero of
Fahrenheit, while in the open grounds, with only a moderate breeze, the
same temperature is almost insupportable. The engineers and firemen of
locomotives, employed on railways running through forests of any
considerable extent, observe that, in very cold weather, it is much
easier to keep up the steam while the engine is passing through the
woods than in the open ground. As soon as the train emerges from the
shelter of the trees the steam-gauge falls, and the stoker is obliged to
throw in a liberal supply of fuel to bring it up again.
Another less frequently noticed fact, due, no doubt, in a great measure
to the immobility of the air, is, that sounds are transmitted to
incredible distances in the unbroken forest. Many instances of this have
fallen under my own observation, and others, yet more striking, have
been related to me by credible and competent witnesses familiar with a
more primitive condition of the Anglo-American world. An acute observer
of natural phenomena, whose childhood and youth were spent in the
interior of one of the newer New England States, has often told me that
when he established his home in the forest, he always distinctly heard,
in still weather, the plash of horses' feet, when they forded a small
brook nearly seven-eighths of a mile from his house, though a portion of
the wood that intervened consisted of a ridge seventy or eighty feet
higher than either the house or the ford.
I have no doubt that, in such cases, the stillness of the air is the
most important element in the extraordinary transmissibilty of sound;
but it must be admitted that the absence of the multiplied, and confused
noises, which accompany human industry in countries thickly peopled by
man, contributes to the same result. We become, by habit, almost
insensible to the familiar and never-resting voices of civilization in
cities and towns; but the indistinguishable drone, which sometimes
escapes even the ear of him who listens for it, deadens and often quite
obstructs the transmission of sounds which would otherwise be clearly
audible. An observer, who wishes to appreciate that hum of civic life
which he cannot analyze, will find an excellent opportunity by placing
himself on the hill of Capo di Monte at Naples, in the line of
prolongation of the street called Spaccanapoli.
It is probably to the stillness of which I have spoken that we are to
ascribe the transmission of sound to great distances at sea in calm
weather. In June, 1853, I and my family were passengers on board a
ship-of-war bound up the Aegean. On the evening of the 27th of that
month, as we were discussing, at the tea-table, some observations of
Humboldt on this subject, the captain of the ship told us that he had
once heard a single gun at sea at the distance of ninety nautical miles.
The next morning, though a light breeze had sprung up from the north,
the sea was of glassy smoothness when we went on deck. As we came up, an
officer told us that he had heard a gun at sunrise, and the conversation
of the previous evening suggested the inquiry whether it could have been
fired from the combined French and English fleet then lying at Beshika
Bay. Upon examination of our position we were found to have been, at
sunrise, ninety sea miles from that point. We continued beating up
northwards, and between sunrise and twelve o'clock meridian of the 28th,
we had made twelve miles northing, reducing our distance from Beshika
Bay to seventy-eight sea miles. At noon we heard several guns so
distinctly that we were able to count the number. On the 29th we came up
with the fleet, and learned from an officer who came on board that a
royal salute had been fired at noon on the 28th, in honor of the day as
the anniversary of the Queen of England's coronation. The report at
sunrise was evidently the morning gun, those at noon the salute.
Such cases are rare, because the sea is seldom still, and the [word in
Greek] rarely silent, over so great a space as ninety or even
seventy-eight nautical miles. I apply the epithet silent to [word in
Greek] advisedly. I am convinced that Aeschylus meant the audible laugh
of the waves, which is indeed of COUNTLESS multiplicity, not the visible
smile of the sea, which, belonging to the great expanse as one
impersonation, is single, though, like the human smile, made up of the
play of many features.] The action of the forest, considered merely as a
mechanical shelter to grounds lying to the leeward of it, might seem to
be an influence of too restricted a character to deserve much notice;
but many facts concur to allow that it is a most important element in
local climate.
It is evident that the effect of the forest, as a mechanical impediment
to the passage of the wind, would extend to a very considerable distance
above its own height, and hence protect while standing, or lay open when
felled, a much larger surface than might at first thought be supposed.
The atmosphere, movable as are its particles, and light and elastic as
are its masses, is nevertheless held together as a continuous whole by
the gravitation of its atoms and their consequent pressure on each
other, if not by attraction between them, and, therefore, an obstruction
which mechanically impedes the movement of a given stratum of air will
retard the passage of the strata above and below it. To this effect may
often be added that of an ascending current from the forest itself,
which must always exist when the atmosphere within the wood is warmer
than the stratum of air above it, and must be of almost constant
occurrence in the case of cold winds, from whatever quarter, because the
still air in the forest is slow in taking up the temperature of the
moving columns and currents around and above it. Experience, in fact,
has shown that mere rows of trees, and even much lower obstructions, are
of essential service in defending vegetation against the action of the
wind. Hardy proposes planting, in Algeria, belts of trees at the
distance of one hundred metres from each other, as a shelter which
experience had proved to be useful in France. [Footnote: Becquerel, Des
Climats, etc., p. 179.] "In the valley of the Rhone," says Becquerel, "a
simple hedge, two metres in height, is a sufficient protection for a
distance of twenty-two metres." [Footnote: Ibid., p. 116. Becquerel's
views have been amply confirmed by recent extensive experiments on the
bleak, stony, and desolate plain of the Cran in the Department of the
Bouches-du-Rhone, which had remained a naked waste from the earliest
ages of history. Belts of trees prove a secure protection even against
the furious and chilly blasts of the Mistral, and in this shelter
plantations of fruit-trees and vegetables, fertilized by the waters and
the slime of the Durance, which are conducted and distributed over the
Cran, thrive with the greatest luxuriance. [Footnote: Surrell, Etude sur
les Torrents, 2d edition, 1872, ii, p. 85.] The mechanical shelter acts,
no doubt, chiefly as a defence against the mechanical force of the wind,
but its uses are by no means limited to this effect. If the current of
air which it resists moves horizontally, it would prevent the access of
cold or parching blasts to the ground for a great distance; and did the
wind even descend at a large angle with the surface, still a
considerable extent of ground would be protected by a forest to the
windward of it.
In the report of a committee appointed in 1836 to examine an article of
the forest code of France, Arago observes; "If a curtain of forest on
the coasts of Normandy and of Brittany were destroyed, these two
provinces would become accessible to the winds from the west, to the
mild breezes of the sea. Hence a decrease of the cold of winter. If a
similar forest were to be cleared on the eastern border of France, the
glacial east wind would prevail with greater strength, and the winters
would become more severe. Thus the removal of a belt of wood would
produce opposite effects in the two regions." [Footnote: Becquerel, Des
Climats, etc., Discours Prelim., vi.]
This opinion receives confirmation from an observation of Dr. Dwight,
who remarks, in reference to the woods of New England: "Another effect
of removing the forest will be the free passage of the winds, and among
them of the southern winds, over the surface. This, I think, has been an
increasing fact within my own remembrance. As the cultivation of the
country has extended further to the north, the winds from the south have
reached distances more remote from the ocean, and imparted their warmth
frequently, and in such degrees as, forty years since, were in the same
places very little known. This fact, also, contributes to lengthen the
summer and to shorten the winter half of the year." [Footnote: Travels,
i., p. 61.]
It is thought in Italy that the clearing of the Apennines has very
materially affected the climate of the valley of the Po. It is asserted
in Le Alpi che cingono l'Italia that: "In consequence of the felling of
the woods on the Apennines, the sirocco prevails greatly on the right
bank of the Po, in the Parmesan territory, and in a part of Lombardy; it
injures the harvests and the vineyards, and sometimes ruins the crops of
the season. To the same cause many ascribe the meteorological changes in
the precincts of Modena and of Reggio. In the communes of these
districts, where formerly straw roofs resisted the force of the winds,
tiles are now hardly sufficient; in others, where tiles answered for
roofs, large slabs of stone are now ineffectual; and in many neighboring
communes the grapes and the grain are swept off by the blasts of the
south and south-west winds."
According to the same authority, the pinery of Porto, near
Ravenna--which is twenty miles long, and is one of the oldest pine woods
in Italy--having been replanted with resinous trees after it was
unfortunately cut, has relieved the city from the sirocco to which it
had become exposed, and in a great degree restored its ancient climate.
[Footnote: Le Alpi che cingono l'Italia, pp. 370, 371.]
The felling of the woods on the Atlantic coast of Jutland has exposed
the soil not only to drifting sands, but to sharp sea-winds, that have
exerted a sensible deteriorating effect on the climate of that
peninsula, which has no mountains to serve at once as a barrier to the
force of the winds, and as a storehouse of moisture received by
precipitation or condensed from atmospheric vapors. [Footnote: Bergsoe,
Reventlovs Virksomhed, ii., p. 125.
The following well-attested instance of a local change of climate is
probably to be referred to the influence of the forest as a shelter
against cold winds. To supply the extraordinary demand for Italian iron
occasioned by the exclusion of English iron in the time of Napoleon I.,
the furnaces of the valleys of Bergamo were stimulated to great
activity. "The ordinary production of charcoal not sufficing to feed the
furnaces and the forges, the woods were felled, the copses cut before
their time, and the whole economy of the forest was deranged. At
Piazzatorre there was such a devastation of the woods, and consequently
such an increased severity of climate, that maize no longer ripened. An
association, formed for the purpose, effected the restoration of the
forest, and maize flourishes again in the fields of Piazzatorre."
--Report by G. Rosa, in Il Politecnico, Dicembre, 1861, p. 614.
Similar ameliorations have been produced by plantations in Belgium. In
an interesting series of articles by Bande, entitled, "Les Cotes de la
Manche," in the Revue des Deux Mondes, I find this statement: "A
spectator, placed on the famous bell-tower of the cathedral of Antwerp,
saw, not long since, on the opposite side of the Schelde, only a vast
desert plain; now he sees a forest, the limits of which are confounded
with the horizon. Let him enter within its shade. The supposed forest is
but a system of regular rows of trees, the oldest of which is not forty
years of age. These plantations have ameliorated the climate which had
doomed to sterility the soil where they are planted. While the tempest
is violently agitating their tops, the air a little below is still, and
sands far more barren than the plateau of La Hague have been
transformed, under their protection, into fertile fields."--Revue des
Deux Mondes, January, 1859, p. 277.] The local retardation of spring, so
much complained of in Italy, France, and Switzerland, and the increased
frequency of late frosts at that season, appear to be ascribable to the
admission of cold blasts to the surface, by the felling of the forests
which formerly both screened it as by a wall, and communicated the
warmth of their soil to the air and earth to the leeward.
Caimi states that since the cutting down of the woods of the Apennines,
the cold winds destroy or stunt the vegetation, and that, in consequence
of "the usurpation of winter on the domain of spring," the district of
Mugello has lost all its mulberries, except the few which find in the
lee of buildings a protection like that once furnished by the forest.
[Footnote: Cenni sulla Importanza e Coltura dei Boschi, p. 31.]
The department of Ardeche, which now contains not a single considerable
wood, has experienced within thirty years a climatic disturbance, of
which the late frosts, formerly unknown in the country, are one of the
most melancholy effects. Similar results have been observed in the plain
of Alsace, in consequence of the denudation of several of the crests of
the Vosges. [Footnote: Clave, Etudes, p. 44.] [Footnote It has been
observed in Sweden that the spring, in many districts where the forests
have been cleared off, now comes on a fortnight later than in the last
century.--Asbjornsen, Om Skovene i norge, p. 101.] Dussard, as quoted by
Ribbe, [Footnote: La Provence au point de vue des Torrents et des
Inondations, p. 10.
Dussard is doubtless historically inaccurate in making the origin of the
mistral so late as the time of Augustus. Diodorus Siculus, who was a
contemporary of Julius Caesar, describes the north-west winds in Gaul as
violent enough to hurl along stones as large as the fist with clouds of
sand and gravel, to strip travellers of their arms and clothing, and to
throw mounted men from their horses. Bibliotheca Historica, lib. v., c.
xxvi. Diodorus, it is true, is speaking of the climate of Gaul in
general, but his description can hardly refer to anything but the
mistral of South-eastern France.] maintains that even the MISTRAL, or
north-west wind, whose chilling blasts are so fatal to tender vegetation
in the spring, "is the child of man, the result of his devastations."
"Under the reign of Augustus," continues he, "the forests which
protected the Cevennes were felled, or destroyed by fire, in mass. A
vast country, before covered with impenetrable woods--powerful obstacles
to the movement and even to the formation of hurricanes--was suddenly
denuded, swept bare, stripped, and soon after, a scourge hitherto
unknown, struck terror over the land from Avignon to the
Bouches-du-Rhone, thence to Marseilles, and then extended its ravages,
diminished indeed by a long career which had partially exhausted its
force, over the whole maritime frontier. The people thought this wind a
curse sent of God. They raised altars to it and offered sacrifices to
appease its rage." It seems, however, that this plague was less
destructive than at present, until the close of the sixteenth century,
when further clearings had removed most of the remaining barriers to its
course. Up to that time, the north-west wind appears not to have
attained to the maximum of specific effect which now characterizes it as
a local phenomenon. Extensive districts, from which the rigor of the
seasons has now banished valuable crops, were not then exposed to the
loss of their harvests by tempests, cold, or drought. The deterioration
was rapid in its progress. Under the Consulate, the clearings had
exerted so injurious an effect upon the climate, that the cultivation of
the olive had retreated several leagues, and since the winters and
springs of 1820 and 1836, this branch of rural industry has been
abandoned in a great number of localities where it was advantageously
pursued before. The orange now flourishes only at a few sheltered points
of the coast, and it is threatened even at Hyeres, where the clearing of
the hills near the town has proved very prejudicial to this valuable
tree.
Marchand informs us that, since the felling of the woods, late spring
frosts are more frequent in many localities north of the Alps; that
fruit-trees thrive no longer, and that it is difficult even to raise
young fruit-trees. [Footnote: Ueber die Entwaldung der Gebirge, p. 28.
Interesting facts and observations on this point will be found in the
valuable Report on the Effects of the Destruction of the Forests in
Wisconsin, by LAPHAM and others, pp. 6, 18, 20.]
Influence of the Forest, considered as Inorganic Matter, on Temperature.
The evaporation of fluids, and the condensation and expansion of vapors
and gases, are attended with changes of temperature; and the quantity of
moisture which the air is capable of containing, and of course, other
things being equal, the evaporation, rise and fall with the thermometer.
The hygroscopical and the thermoscopical conditions of the atmosphere
are, therefore, inseparably connected as reciprocally dependent
quantities, and neither can be fully discussed without taking notice of
the other. The leaves of living trees exhale enormous quantities of gas
and of aqueous vapor, and they largely absorb gases, and, under certain
conditions, probably also water. Hence they affect more or less
powerfully the temperature as well as the humidity of the air. But the
forest, regarded purely as inorganic matter, and without reference to
its living processes of absorption and exhalation of gases and of water,
has, as an absorbent, a radiator and a conductor of heat, and as a mere
covering of the ground, an influence on the temperature of the air and
the earth, which may be considered by itself.
Absorbing and Emitting Surface.
A given area of ground, as estimated by the every-day rule of
measurement in yards or acres, presents always the same apparent
quantity of absorbing, radiating, and reflecting surface; but the real
extent of that surface is very variable, depending, as it does, upon its
configuration, and the bulk and form of the adventitious objects it
bears upon it; and, besides, the true superficies remaining the same,
its power of absorption, radiation, reflection, and conduction of heat
will be much affected by its consistence, its greater or less humidity,
and its color, as well as by its inclination of plane and exposure. An
acre of clay, rolled hard and smooth, would have great reflecting power,
but its radiation would be much increased by breaking it up into clods,
because the actually exposed surface would be greater, though the
outline of the field remained the same. The inequalities, natural or
artificial, which always occur in the surface of ordinary earth, affect
in the same way its quantity of superficies acting upon the temperature
of the atmosphere, and acted on by it, though the amount of this action
and reaction is not susceptible of measurement.
Analogous effects are produced by other objects, of whatever form or
character, standing or lying upon the earth, and no solid can be placed
upon a flat piece of ground, without itself exposing a greater surface
than it covers. This applies, of course, to forest trees and their
leaves, and indeed to all vegetables, as well as to other prominent
bodies. If we suppose forty trees to be planted on an acre, one being
situated in the centre of every square of two rods the side, and to grow
until their branches and leaves everywhere meet, it is evident that,
when in full foliage, the trunks, branches, and leaves would present an
amount of thermoscopic surface much greater than that of an acre of bare
earth; and besides this, the fallen leaves lying scattered on the
ground, would somewhat augment the sum-total. [Footnote: "The Washington
elm at Cambridge--a tree of no extraordinary size--was some years ago
estimated to produce a crop of seven millions of leaves, exposing a
surface of two hundred thousand square feet, or about five acres of
foliage."--Gray, First Lessons in Botany and Vegetable Physiology.] On
the other hand, the growing leaves of trees generally form a succession
of stages, or, loosely speaking, layers, corresponding to the annual
growth of the branches, and more or less overlying each other. This
disposition of the foliage interferes with that free communication
between sun and sky above, and leaf-surface below, on which the amount
of radiation and absorption of light depends. From all these
considerations, it appears that though the effective thermoscopic
surface of a forest in full leaf does not exceed that of bare ground in
the same proportion as does its measured superficies, yet the actual
quantity of area capable of receiving and emitting heat must be greater
in the former than in the latter case. [Footnote: See, on this particular
point, and on the general influence of the forest on temperature,
Humboldt, Ansichten der Natur, i., 158.]
It must further be remembered that the form and texture of a given
surface are important elements in determining its thermoscopic
character. Leaves are porous, and admit air and light more or less
freely into their substance; they are generally smooth and even glazed
on one surface; they are usually covered on one or both sides with
spicula, and they very commonly present one or more acuminated points in
their outline--all circumstances which tend to augment their power of
emitting heat by reflection or radiation. Direct experiment on growing
trees is very difficult, nor is it in any case practicable to
distinguish how far a reduction of temperature produced by vegetation is
due to radiation, and how far to exhalation of the gaseous and watery
fluids of the plant; for both processes usually go on together. But the
frigorific effect of leafy structure is well observed in the deposit of
dew and the occurrence of hoarfrost on the foliage of grasses and other
small vegetables, and on other objects of similar form and consistence,
when the temperature of the air a few feet above has not been brought
down to the dew-point, still less to 32 degrees, the degree of cold
required to congeal dew to frost. [Footnote: The leaves and twigs of
plants may be reduced by radiation to a temperature lower than that of
the ambient atmosphere, and even be frozen when the air in contact with
them is above 32 degrees. Their temperature may be communicated to the
dew deposited on them and thus this dew be converted into frost when
globules of watery fluid floating in the atmosphere near them, in the
condition of fog or vapor, do not become congealed.
It has long been known that vegetables can be protected against frost by
diffusing smoke through the atmosphere above them. This method has been
lately practised in France on a large scale: vineyards of forty or fifty
acres have been protected by placing one or two rows of pots of burning
coal-tar, or of naphtha, along the north side of the vineyard, and thus
keeping up a cloud of smoke for two or three hours before and after
sunrise. The expense is said to be small, and probably it might be
reduced by mixing some less combustible substance, as earth, with the
fluid, and thus checking its too rapid burning.
The radiating and refrigerating power of objects by no means depends on
their form alone. Melloni cut sheets of metal into the shape of leaves
and grasses, and found that they produced little cooling effect, and
were not moistened under atmospheric conditions which determined a
plentiful deposit of dew on the leaves of vegetables.]
We are also to take into account the action of the forest as a conductor
of heat between the atmosphere and the earth. In the most important
countries of America and Europe, and especially in those which have
suffered most from the destruction of the woods, the superficial strata
of the earth are colder in winter, and warmer in summer, than those a
few inches lower, and their shifting temperature approximates to the
atmospheric mean of the respective seasons. The roots of large trees
penetrate beneath the superficial strata, and reach earth of a nearly
constant temperature, corresponding to the mean for the entire year. As
conductors, they convey the heat of the atmosphere to the earth when the
earth is colder than the air, and transmit it in the contrary direction
when the temperature of the earth is higher than that of the atmosphere.
Of course, then, as conductors, they tend to equalize the temperature of
the earth and the air.
In countries where the questions I am considering have the greatest
practical importance, a very large proportion, if not a majority, of the
trees are of deciduous foliage, and their radiating as well as their
shading surface is very much greater in summer than in winter. In the
latter season, they little obstruct the reception of heat by the ground
or the radiation from it; whereas, in the former, they often interpose a
complete canopy between the ground and the sky, and materially interfere
with both processes.
Dead Products of Trees.
Besides this various action of standing trees, considered as inorganic
matter, the forest exercises, by the annual moulting of its foliage,
still another influence on the temperature of the earth, and,
consequently, of the atmosphere which rests upon it. If we examine the
constitution of the superficial soil in a primitive or an old and
undisturbed artificially planted wood, we find, first, a deposit of
undecayed leaves, twigs, and seeds, lying in loose layers on the
surface; then, more compact beds of the same materials in incipient,
and, as we descend, more and more advanced, stages of decomposition;
then, a mass of black mould, in which traces of organic structure are
hardly discoverable except by microscopic examination; then, a stratum
of mineral soil, more or less mixed with vegetable matter carried down
into it by water, or resulting from the decay of roots; and, finally,
the inorganic earth or rock itself. Without this deposit of the dead
products of trees, this latter would be the superficial stratum, and as
its powers of absorption, radiation, and conduction of heat would differ
essentially from those of the layers with which it has been covered by
the droppings of the forest, it would act upon the temperature of the
atmosphere, and be acted on by it, in a very different way from the
leaves and mould which rest upon it. Dead leaves, still entire, or
partially decayed, are very indifferent conductors of light, and,
therefore, though they diminish the warming influence of the summer sun
on the soil below them, they, on the other hand, prevent the escape of
heat from that soil in winter, and, consequently, in cold climates, even
when the ground is not covered by a protecting mantle of snow, the earth
does not freeze to as great a depth in the wood as in the open field.
Specific Heat.
Trees, considered as organisms, produce in themselves, or in the air, a
certain amount of heat, by absorbing and condensing atmospheric gases,
and they exert an opposite influence by absorbing water and exhaling it
in the form of vapor; but there is still another mode by which their
living processes may warm the air around them, independently of the
thermometric effects of condensation and evaporation. The vital heat of
a dozen persons raises the temperature of a room. If trees possess a
specific temperature of their own, an organic power of generating heat
like that with which the warm-blooded animals are gifted, though by a
different process, a certain amount of weight is to be ascribed to this
element in estimating the action of the forest upon atmospheric
temperature.
Boussingault remarks: "In many flowers there has been observed a very
considerable evolution of heat, at the approach of fecundation. In
certain arums the temperature rises to 40 degrees or 50 degrees Cent. [=
104 degrees or 122 degrees Fahr.] It is very probable that this
phenomenon in general, and varies only in the intensity which it is
manifested." [Footnote: Economie Rurale, i., p. 22.]
If we suppose the fecundation of the flowers of forest trees to be
attended with a tenth only of this calorific power, they could not fail
to exert an important influence on the warmth of the atmospheric strata
in contact with them.
Experiments by Meguscher, in Lombardy, led that observer to conclude
"that the wood of a living tree maintains a temperature of + 12 degrees
or 18 degrees Cent. [= 54 degrees, 56 degrees Fahr.] when the
temperature of the air stands at 3 degrees, 7 degrees, and 8 degrees [=
37 degrees, 46 degrees, 47 degrees F.] above zero, and that the internal
warmth of the tree does not rise and fall in proportion to that of the
atmosphere. So long as the latter is below 18 degrees [= 67 degrees
Fahr.], that of the tree is always the highest; but if the temperature
of the air rises to 18 degrees, that of the vegetable growth is the
lowest. Since then, trees maintain at all seasons a constant mean
temperature of 12 degrees [= 54 degrees Fahr.], it is easy to see why
the air in contact with the forest must be warmer in winter, cooler in
summer than in situations where it is deprived of that influence."
[Footnote: Memoria Sur Boschi Della Lombardia, p. 45. The results of
recent experiments by Becquerel do not accord with those obtained by
Meguscher, and the former eminent physicist holds that "a tree is warmed
in the air like any inert body." At the same time he asserts, as a fact
well ascertained by experiment, that "vegetables possess in themselves
the power or resisting extreme cold for a certain length of time,....
and hence it is believed that there may exist in the organism of plants
a force, independent of the conduction of caloric, which resists a
degree of cold above the freezing-point." In a following page he cites
observations made by Bugeaud, under the parallel of 58 degrees N. L.,
between the months of November and June, during most of which time, of
course, vegetable life was in its deepest lethargy. Bugeaud found that
when the temperature of the air was at -34.60 degrees, that of a poplar
was only at -29.70 degrees, which certainly confirms the doctrine that
trees exercise a certain internal resistance against cold.]
Professor Henry says: "As a general deduction from chemical and
mechanical principles, we think no change of temperature is ever
produced where the actions belonging to one or both of these principles
are not present. Hence, in midwinter, when all vegetable functions are
dormant, we do not believe that any heat is developed by a tree, or that
its interior differs in temperature from its exterior further than it is
protected from the external air. The experiments which have been made on
this point, we think, have been directed by a false analogy. During the
active circulation of the sap and the production of new tissue,
variations of temperature belonging exclusively to the plant may be
observed; but it is inconsistent with general principles that heat
should be generated where no change is taking place." [Footnote: United
States Patent Office Report for 1857, p. 504.]
There can be no doubt that moisture is given, out by trees and
evaporated in extremely cold winter weather, and unless new fluid were
supplied from the roots by the exercise of some vital function, the tree
would be exhausted of its juices before winter was over. But this is not
observed to be the fact, and, though the point is disputed, respectable
authorities declare that "wood felled in the depth of winter is the
heaviest and fullest of sap." [Footnote: Rossmassler, Der Wald, p. 158.]
Warm weather in winter, of too short continuance to affect the
temperature of the ground sensibly, stimulates a free flow of sap in the
maple. Thus, in the last week of December, 1862, and the first week of
January, 1863, sugar was made from that tree in various parts of New
England. "A single branch of a tree, admitted into a warm room in winter
through an aperture in a window, opened its buds and developed its
leaves, while the rest of the tree in the external air remained in its
winter sleep." [Footnote: Ibid., p. 160.] Like facts are matter of
every-day observation in graperies where the vine is often planted
outside the wall, the stem passing through an aperture into the warm
interior. The roots, of course, stand in ground of the ordinary winter
temperature, but vegetation is developed in the branches at the pleasure
of the gardener. The roots of forest trees in temperate climates remain,
for the most part, in a moist soil, of a temperature not much below the
annual mean, through the whole winter; and we cannot account for the
uninterrupted moisture of the tree, unless we suppose that the roots
furnish a constant supply of water. Atkinson describes a ravine in a
valley in Siberia, which was filled with ice to the depth of twenty-five
feet. Poplars were growing in this ice, which was thawed to the distance
of some inches from the stem. But the surface of the soil beneath it
must have remained still frozen, for the holes around the trees were
full of water resulting from its melting, and this would have escaped
below if the ground had been thawed. In this case, although the roots
had not thawed the thick covering of earth above them, the trunks must
have melted the ice in contact with them. The trees, when observed by
Atkinson, were in full leaf, but it does not appear at what period the
ice around their stems had melted.
From these facts, and others of the like sort, it would seem that "all
vegetable functions are" not absolutely "dormant in winter, and,
therefore, that trees may give out SOME heat even at that season."
[Footnote: All evergreens, even the broad-leaved trees, resist frosts of
extraordinary severity better than the deciduous trees of the same
climates. Is not this because the vital processes of trees of persistent
foliage are less interrupted during winter than those of trees which
annually shed their leaves, and that therefore more organic heat is
developed?
In crossing Mont Cenis in October, 1869, when the leaves of the larches
on the northern slope and near the top of the mountain were entirely
dead and turned brown, I observed that these trees were completely white
with hoar-frost. It was a wonderful sight to see how every leaf was
covered with a delicate deposit of frozen aqueous vapor, which gave the
effect of the most brilliant silver. On the other band, the evergreen
coniferae, which were growing among the larches, and therefore in the
same conditions of exposure, were almost entirely free from frost. The
contrast between the verdure of the leaves of the evergreens and the
crystalline splendor of those of the larches was strikingly beautiful.
Was this fact due to a difference in the color and structure of the
leaves, or rather is it a proof of a vital force of resistance to cold
in the living foliage of the evergreen tree The low temperature of air
and soil at which, in the frigid zone, as well as in warmer latitudes
under special circumstances, the processes of vegetation go on, seems to
necessitate the supposition that all the manifestations of vegetable
life are attended with an evolution of heat. In the United States it is
common to protect ice, in ice-houses, by a covering of straw, which
naturally sometimes contains kernels of grain. These often sprout, and
even throw out roots and leaves to a considerable length, in a
temperature very little above the freezing-point. Three or four years
since I saw a lump of very clear and apparently solid ice, about eight
inches long by six thick, on which a kernel of grain had sprouted in an
ice-house, and sent half a dozen or more very slender roots into the
pores of the ice and through the whole length of the lump. The young
plant must have thrown out a considerable quantity of heat; for though
the ice was, as I have said, otherwise solid, the pores through which
the roots passed were enlarged to perhaps double the diameter of the
fibres, but still not so much as to prevent the retention of water in
them by capillary attraction.]
It does not appear that observations have been made on the special point
of the development of heat in forest trees during florification, or at
any other period of intense vital action; and hence an important element
in the argument remains undetermined. The "circulation of the sap"
commences at a very early period in the spring, and the temperature of
the air in contact with trees may then be sufficiently affected by heat
evolved in the vital processes of vegetation, to raise the thermometric
mean of wooded countries for that season, and, of course, for the year.
The determination of this point is of much greater importance to
vegetable physiology than the question of the winter temperature of
trees, because a slight increment of heat in the trees of a forest might
so affect the atmosphere in contact with them as to make possible the
growing of many plants in or near the wood which could not otherwise he
reared in that climate.
The evaporation of the juices of trees and other plants is doubtless
their most important thermoscopic function, and as recent observations
lead to the conclusion that the quantity of moisture exhaled by
vegetables has been hitherto underrated, we must ascribe to this element
a higher value than has been usually assigned to it as a meteorological
influence.
The exhalation and evaporation of the juices of trees, by whatever
process effected, take up atmospheric heat and produce a proportional
refrigeration. This effect is not less real, though to common
observation less sensible, in the forest than in meadow or pasture land,
and it cannot be doubted that the local temperature is considerably
affected by it. But the evaporation that cools the air diffuses through
it, at the same time, a medium which powerfully resists the escape of
heat from the earth by radiation. Visible vapors, fogs and clouds, it is
well known, prevent frosts by obstructing radiation, or rather by
reflecting back again the heat radiated by the earth, just as any
mechanical screen would do. On the other hand, fogs and clouds intercept
the rays of the sun also, and hinder its heat from reaching the earth.
The invisible vapors given out by leaves impede the passage of heat
reflected and radiated by the earth and by all terrestrial objects, bat
oppose much less resistance to the transmission of direct solar heat,
and indeed the beams of the sun seem more scorching when received
through clear air charged with uncondensed moisture than after passing
through a dry atmosphere. Hence the reduction of temperature by the
evaporation of moisture from vegetation, though sensible, is less than
it would be if water in the gaseous state were as impervious to heat
given out by the sun as to that emitted by terrestrial objects.
Total Influence of the Forest on Temperature.
It has not yet been found practicable to measure, sum up, and equate the
total influence of the forest, its processes and its products, dead and
living, upon temperature, and investigators differ much in their
conclusions on this subject. It seems probable that in every particular
case the result is, if not determined, at least so much modified by
local conditions which are infinitely varied, that no general formula is
applicable to the question. In the report to which I referred on page
163, Gay-Lussac says; "In my opinion we have not yet any positive proof
that the forest has, in itself, any real influence on the climate of a
great country, or of a particular locality. By closely examining the
effects of clearing off the woods, we should perhaps find that, far from
being an evil, it is an advantage; but these questions are so
complicated when they are examined in a climatological point of view,
that the solution of them is very difficult, not to say impossible."
Becquerel, on the other hand, considers it certain that in tropical
climates the destruction of the forests is accompanied with an elevation
of the mean temperature, and he thinks it highly probable that it has
the same effect in the temperate zones. The following is the substance
of his remarks on this subject: "Forests act as frigorific causes in
three ways:
"1. They shelter the ground against solar irradiation and maintain a
greater humidity.
"2. They produce a cutaneous transpiration by the leaves.
"3. They multiply, by the expansion of their branches, the surfaces
which are cooled by radiation.
"These three causes acting with greater or less force, we must, in the
study of the climatology of a country, take into account the proportion
between the area of the forests and the surface which is bared of trees
and covered with herbs and grasses.
"We should be inclined to believe, a priori, according to the foregoing
considerations, that the clearing of the woods, by raising the
temperature and increasing the dryness of the air, ought to react on
climate. There is no doubt that, if the vast desert of the Sahara were
to become wooded in the course of ages, the sands would cease to be
heated as much as at the present epoch, when the mean temperature is
twenty-nine degrees [Centigrade, = 85 degrees Fahr.]. In that case, the
ascending currents of warm air would cease, or be less warm, and would
not contribute, by descending in our latitudes, to soften the climate of
Western Europe. Thus the clearing of a great country may react on the
climates of regions more or less remote from it.
"The observations by Boussingault leave no doubt on this point. This
writer determined the mean temperature of wooded and of cleared points,
under the same latitude, and at the same elevation above the sea, in
localities comprised between the eleventh degree of north and the fifth
degree of south latitude, that is to say, in the portion of the tropics
nearest to the equator, and where radiation tends powerfully during the
night to lower the temperature under a sky without clouds." [Footnote:
Becquerel, Des Climats, etc., pp. 139-141.]
The result of these observations, which has been pretty generally
adopted by physicists, is that the mean temperature of cleared land in
the tropics appears to be about one degree Centigrade, or a little less
than two degrees of Fahrenheit, above that of the forest. On page 147 of
the volume just cited, Becquerel argues that, inasmuch as the same and
sometimes a greater difference is found in favor of the open ground, at
points within the tropics so elevated as to have a temperate or even a
polar climate, we must conclude that theforests in Northern America
exert a refrigerating influence equally powerful. But the conditions of
the soil are so different in the two regions compared, that I think we
cannot, with entire confidence, reason from the one to the other, and it
is much to be desired that observations be made on the summer and winter
temperature of both the air and the ground in the depths of the North
American forests, before it is too late.
Recent inquiries have introduced a new element into the problem of the
influence of the forest on temperature, or rather into the question of
the thermometrical effects of its destruction. I refer to the
composition of the soil in respect to its hygroscopicity or aptitude to
absorb humidity, whether in a liquid or a gaseous form, and to the
conducting power of the particles of which it is composed. [Footnote:
Composition, texture, and color of soil are important elements to be
considered in estimating the effects of the removal of the forest upon
its thermoscopic action. "Experience has proved," says Becquerel, "that
when the soil is bared, it becomes more or less heated [by the rays of
the sun] according to the nature and the color of the particles which
compose it, and according to its humidity, and that, in the
refrigeration resulting from radiation, we must take into the account
the conducting power of those particles also. Other things being equal,
siliceous and calcareous sands, compared in equal volumes with different
argillaceous earths, with calcareous powder or dust, with humus, with
arable and with garden earth, are the soils which least conduct heat. It
is for this reason that sandy ground, in summer, maintains a high
temperature even during the night. We may hence conclude that when a
sandy soil is stripped of wood, the local temperature will be raised.
After the sands follow successively argillaceous, arable, and garden
ground, then humus, which occupies the lowest rank.
"The retentive power of humus is but half as great as that of calcareous
sand. We will add that the power or retaining heat is proportional to
the density. It has also a relation to the magnitude of the particles.
It is for this reason that ground covered with siliceous pebbles cools
more slowly than siliceous sand, and that pebbly soils are best suited
to the cultivation of the vine, because they advance the ripening of the
grape more rapidly than chalky and clayey earths, which cool quickly.
Hence we see that in examining the calorific effects of clearing
forests, it is important to take into account the properties of the soil
laid bare."--Becquerel, Des Climats et des Sols boises, p. 137.]
The hygroscopicity of humus or vegetable earth is much greater than that
of any mineral soil, and consequently forest ground, where humus
abounds, absorbs the moisture of the atmosphere more rapidly and in
larger proportion than common earth. The condensation of vapor by
absorption develops heat, and consequently elevates the temperature of
the soil which absorbs it, together with that of air in contact with the
surface. Von Babo found the temperature of sandy ground thus raised from
68 degrees to 80 degrees F., that of soil rich in humus from 68 degrees
to 88 degrees. The question of the influence of the woods on temperature
does not, in the present state of our knowledge, admit of precise
solution, and, unhappily, the primitive forests are disappearing so
rapidly before the axe of the woodman, that we shall never be able to
estimate with accuracy the climatological action of the natural wood,
though all the physical functions of artificial plantations will,
doubtless, one day be approximately known.
But the value of trees as a mechanical screen to the soil they cover,
and often to ground far to the leeward of them, is most abundantly
established, and this agency alone is important enough to justify
extensive plantation in all countries which do not enjoy this
indispensable protection.
Influence of Forests as Inorganic on the Humidity of the Air and the
Earth.
The most important hygroscopic as well as thermoscopic influence of the
forest is, no doubt, that which it exercises on the humidity of the air
and the earth, and this climatic action it exerts partly as dead, partly
as living matter. By its interposition as a curtain between the sky and
the ground it both checks evaporation from the earth, and mechanically
intercepts a certain proportion of the dew and the lighter showers,
which would otherwise moisten the surface of the soil, and restores it
to the atmosphere by exhalation; [Footnote: Mangotti had observed and
described, in his usual picturesque way, the retention of rain-water by
the foliage and bark of trees, but I do not know that any attempts were
made to measure the quantity thus intercepted before the experiments of
Becquerel, communicated to the Academy of Sciences in 1866. These
experiments embraced three series of observations continued respectively
for periods of a year, a month, and two days. According to Becquerel's
measurements, the quantity falling on bare and on wooded soil
respectively was as 1 to 0.07; 1 to 0.5; and 1 to 0.6, or, in other
words, he found that only from five-tenths to sixty-seven hundredths of
the precipitation reached the ground.--Comptes Rendus de l'Academie des
Sciences, 1866. It seemed, indeed, improbable that in rain-storms which
last not hours but whole days in succession, so large a proportion of
the downfall should continue to be intercepted by forest vegetation
after the leaves, the bark, and the whole framework of the trees were
thoroughly wet, but the conclusions of this eminent physicist appear to
have been generally accepted until the very careful experiments of
Mathieu at the Forest-School of Nancy were made known. The observations
of Mathieu were made in a plantation of deciduous trees forty-two years
old, and were continued through the entire years 1866, 1867, and 1868.
The result was that the precipitation in the wood was to that in an open
glade of several acres near the forest station as 043 to 1,000, and the
proportion in each of the three years was nearly identical. According to
Mathieu, then, only 57 thousandths or 5.7 per cent of the precipitation
is intercepted by trees.--Surrell, Etude sur les Torrents, 2d ed., ii.,
p. 98.
By order of the Direction of the Forests of the Canton of Berne, a
series of experiments on this subject was commenced at the beginning of
the year 1869. During the first seven months of the year (the reports
for which alone I have seen), including, of course, the season when the
foliage is most abundant, as well as that when it is thinnest, the
pluviometers in the woods received only fifteen per cent less than those
in the open grounds in the vicinity.--Risler, in Revue des Eaux et
Forets, of 10th January, 1870.] while in heavier rains, the large drops
which fall upon the leaves and branches are broken into smaller ones,
and consequently strike the ground with less mechanical force, or are
perhaps even dispersed into vapor without reaching it. [Footnote: We are
not, indeed, to suppose that the condensation of vapor and the
evaporation of water are going on in the same stratum of air at the same
time, or, in other words, that vapor is condensed into rain-drops, and
rain-drops evaporated, under the same conditions; but rain formed in one
stratum may fall through another, where vapor would not be condensed.
Two saturated strata of different temperatures may be brought into
contact in the higher regions, and discharge large rain-drops, which, it
not divided by some obstruction, will reach the ground, though passing
through strata which would vaporize them if they were in a state of more
minute division.]
The vegetable mould, resulting from the decomposition of leaves and of
wood, serves as a perpetual mulch to forest-soil by carpeting the ground
with a spongy covering which obstructs the evaporation from the mineral
earth below, [Footnote: The only direct experiments known to me on the
evaporation from the SURFACE of the forest are those of
Mathieu.--Surrell, Etude sur les Torrents, 2d ed., ii, p. 99.
These experiments were continued from March to December, inclusive, of
the year 1868. It was found that during those months the evaporation
from a recipient placed on the ground in a plantation of deciduous trees
sixty-two years old, was less than one-fifth of that from a recipient of
like form and dimensions placed in the open country.] drinks up the
rains and melting snows that would otherwise flow rapidly over the
surface and perhaps be conveyed to the distant sea, and then slowly
gives out, by evaporation, infiltration, and percolation, the moisture
thus imbibed. The roots, too, penetrate far below the superficial soil,
conduct water along their surface to the lower depths to which they
reach, and thus by partially draining the superior strata, remove a
certain quantity of moisture out of the reach of evaporation. The Forest
as Organic.
These are the principal modes in which the humidity of the atmosphere is
affected by the forest regarded as lifeless matter. Let us inquire how
its organic processes act upon this meteorological element. The
commonest observation shows that the wood and bark of living trees are
always more or less pervaded with watery and other fluids, one of which,
the sap, is very abundant in trees of deciduous foliage when the buds
begin to swell and the leaves to develop themselves in the spring. This
fluid is drawn principally, if not entirely, from the ground by the
absorbent action of the roots, for though Schacht and some other eminent
botanical physiologists have maintained that water is absorbed by the
leaves and bark of trees, yet most experiments lead to the contrary
result, and it is now generally held that no water is taken in by the
pores of vegetables. Late observations by Cailletet, in France, however,
tend to the establishment of a new doctrine on this subject which solves
many difficulties and will probably be accepted by botanists as
definitive. Cailletet finds that under normal conditions, that is, when
the soil is humid enough to supply sufficient moisture through the
roots, no water is absorbed by the leaves, buds, or bark of plants, but
when the roots are unable to draw from the earth the requisite quantity
of this fluid, the vegetable pores in contact with the atmosphere absorb
it from that source.
Popular opinion, indeed, supposes that all the vegetable fluids, during
the entire period of growth, are drawn from the bosom of the earth, and
that the wood and other products of the tree are wholly formed from
matter held in solution in the water abstracted by the roots from the
ground. This is an error, for the solid matter of the tree, in a certain
proportion not important to our present inquiry, is received from the
atmosphere in a gaseous form, through the pores of the leaves and of the
young shoots, and, as we have just seen, moisture is sometimes supplied
to trees by the atmosphere. The amount of water taken up by the roots,
however, is vastly greater than that imbibed through the leaves and
bark, especially at the season when the sap is most abundant, and when
the leaves are yet in embryo. The quantity of water thus received from
the air and the earth, in a single year, even by a wood of only a
hundred acres, is very great, though experiments are wanting to furnish
the data for even an approximate estimate of its measure; for only the
vaguest conclusions can be drawn from the observations which have been
made on the imbibition and exhalation of water by trees and other plants
reared in artificial conditions diverse from those of the natural
forest. [Footnote: The experiments of Hales and others on the absorption
and exhalation of vegetables are of high physiological interest; but
observations on sunflowers, cabbages, hops, and single branches of
isolated trees, growing in artificially prepared soils and under
artificial conditions, furnish no trustworthy data for computing the
quantity of water received and given off by the natural wood.]
Flow of Sap.
The amount of sap which can be withdrawn from living trees furnishes,
not indeed a measure of the quantity of water sucked up by their roots
from the ground--for we cannot extract from a tree its whole
moisture--but numerical data which may aid the imagination to form a
general notion of the powerful action of the forest as an absorbent of
humidity from the earth.
The only forest-tree known to Europe and North America, the sap of which
is largely enough applied to economical uses to have made the amount of
its flow a matter of practical importance and popular observation, is
the sugar maple, Acer saccharinum, of the Anglo-American Provinces and
States. In the course of a single "sugar season," which lasts ordinarily
from twenty-five to thirty days, a sugar maple two feet in diameter will
yield not less than twenty gallons of sap, and sometimes much more.
[Footnote: Emerson (Trees of Massachusetts. p. 403) mentions a maple six
feet in diameter, as having yielded a barrel, or thirty-one and a half
gallons, of sap in twenty-four hours, and another, the dimensions of
which are not stated, as having yielded one hundred and seventy-five
gallons in the course of the season.
The Cultivator, an American agricultural journal, for June, 1842, states
that twenty gallons of sap were drawn in eighteen hours from a single
maple, two and a half feet in diameter, in the town of Warner, New
Hampshire, and the truth of this account has been verified by personal
inquiry made in my behalf. This tree was of the original forest growth,
and had been left standing when the ground around it was cleared. It was
tapped only every other year, and then with six or eight incisions. Dr.
Williams (History of Vermont, i., p. 01) says: "A man much employed in
milking maple sugar, found that, for twenty-one days together, a
maple-tree discharged seven and a half gallons per day."
An intelligent correspondent, of much experience in the manufacture of
maple sugar, writes me that a second-growth maple, of about two feet in
diameter, standing in open ground, tapped with four incisions, has, for
several seasons, generally run eight gallons per day in fair weather. He
speaks of a very large tree, from which sixty gallons were drawn in the
course of a season, and of another, something more than three feet
through, which made forty-two pounds of wet sugar, and must have yielded
not less than one hundred and fifty gallons.] This, however, is but a
trifling proportion of the water abstracted from the earth by the roots
during this season; for all this fluid runs from two or three incisions
or auger-holes, so narrow as to intercept the current of comparatively
few sap vessels, and besides, experience shows that large as is the
quantity withdrawn from the circulation, it is relatively too small to
affect very sensibly the growth of the tree. [Footnote: Tapping does not
check the growth, but does injure the quality of the wood of maples. The
wood of trees often tapped is lighter and less dense than that of trees
which have not been tapped, and gives less heat in burning. No
difference has been observed in the bursting of the buds of tapped and
untapped trees.] The number of large maple-trees on an acre is
frequently not less than fifty, [Footnote: Dr. Rush, in a letter to
Jefferson, states the number of maples fit for tapping on an acre at
from thirty to fifty. "This," observes my correspondent, "is correct
with regard to the original growth, which is always more or less
intermixed with other trees; but in second growth, composed of maples
alone, the number greatly exceeds this. I have had the maples on a
quarter of an acre, which I thought about an average of second-growth
'maple orchards,' counted. The number was found to be fifty-two, of
which thirty-two were ten inches or more in diameter, and, of course,
large enough to tap. This gives two hundred and eight trees to the acre,
one hundred and twenty-eight of which were of proper size for tapping."]
and of course the quantity of moisture abstracted from the soil by this
tree alone is measured by thousands of gallons to the acre. The sugar
orchards, as they are called, contain also many young maples too small
for tapping, and numerous other trees--two of which, at least, the black
birch, Betula lenta, and yellow birch, Betula excelsa, both very common
in the same climate, are far more abundant in sap than the maple
[Footnote: The correspondent already referred to informs me that a black
birch, tapped about noon with two incisions, was found the next morning
to have yielded sixteen gallons. Dr. Williams (History of Vermont, i.,
p. 91) says: "A large birch, tapped in the spring, ran at the rate of
five gallons an hour when first tapped. Eight or nine days after, it was
found to run at the rate of about two and a half gallons an hour, and at
the end of fifteen days the discharge continued in nearly the same
quantity. The sap continued to flow for four or five weeks, and it was
the opinion of the observers that it must have yielded as much as sixty
barrels [l,800 gallons]."]--are scattered among the sugar-trees; for the
North American native forests are remarkable for the mixture of their
crops. The sap of the maple, and of other trees with deciduous leaves
which grow in the same climate, flows most freely in the early spring,
and especially in clear weather, when the nights are frosty and the days
warm; for it is then that the melting snows supply the earth with
moisture in the justest proportion, and that the absorbent power of the
roots is stimulated to its highest activity.
When the buds are ready to burst, and the green leaves begin to show
themselves beneath their scaly covering, the ground has become drier,
the absorption by the roots is diminished, and the sap, being
immediately employed in the formation of the foliage, can be extracted
from the stem in only small quantities.
Absorption and Exhalation by Foliage.
The leaves now commence the process of absorption, and imbibe both
uncombined gases and an unascertained but probably inconsiderable
quantity of aqueous vapor from the humid atmosphere of spring which
bathes them.
The organic action of the tree, as thus far described, tends to the
desiccation of air and earth; but when we consider what volumes of water
are daily absorbed by a large tree, and how small a proportion of the
weight of this fluid consists of matter which, at the period when the
flow of sap is freest, enters into new combinations, and becomes a part
of the solid framework of the vegetable, or a component of its deciduous
products, it becomes evident that the superfluous moisture must somehow
be carried back again almost as rapidly as it flows into the tree. At
the very commencement of vegetation in spring, some of this fluid
certainly escapes through the buds, the nascent foliage, and the pores
of the bark, and vegetable physiology tells us that there is a current
of sap towards the roots as well as from them. [Footnote: "The
elaborated sap, passing out of the leaves, is received into the inner
bark, . . . and a part of what descends finds its way even to the ends
of the roots, and is all along diffused laterally into the stem, where
it meets and mingles with the ascending crude sap or raw material. So
there is no separate circulation of the two kinds of sap; and no crude
sap exists separately in any part of the plant. Even in the root, where
it enters, this mingles at once with some elaborated sap already
there."--Gray, How Plants Grow, Section 273.]
I do not know that the exudation of water into the earth, through the
bark or at the extremities of these latter organs, has been proved, but
the other known modes of carrying off the surplus do not seem adequate
to dispose of it at the almost leafless period when it is most
abundantly received, and it is possible that the roots may, to some
extent, drain as well as flood the water-courses of their stem. Later in
the season the roots absorb less, and the now developed leaves exhale an
increased quantity of moisture into the air. In any event, all the water
derived by the growing tree from the atmosphere and the ground is parted
with by transpiration or exudation, after having surrendered to the
plant the small proportion of matter required for vegetable growth which
it held in solution or suspension. [Footnote: Ward's tight glazed cases
for raising and especially for transporting plants, go far to prove that
water only circulates through vegetables, and is again and again
absorbed and transpired by organs appropriated to these functions.
Seeds, growing grasses, shrubs, or trees planted in proper earth,
moderately watered and covered with a glass bell or close frame of
glass, live for months, and even years, with only the original store of
air and water. In one of Ward's early experiments, a spire of grass and
a fern, which sprang up in a corked bottle containing a little moist
earth introduced as a bed for a snail, lived and flourished for eighteen
years without a new supply of either fluid. In these boxes the plants
grow till the enclosed air is exhausted of the gaseous constituents of
vegetation, and till the water has yielded up the assimilable matter it
held in solution, and dissolved and supplied to the roots the nutriment
contained in the earth in which they are planted. After this, they
continue for a long time in a state of vegetable sleep, but if fresh air
and water be introduced into the cases, or the plants be transplanted
into open ground, they rouse themselves to renewed life, and grow
vigorously, without appearing to have suffered from their long
imprisonment. The water transpired by the leaves is partly absorbed by
the earth directly from the air, partly condensed on the glass, along
which it trickles down to the earth, enters the roots again, and thus
continually repeats the circuit. See Aus der Natur, 21, B. S. 537.] The
hygrometrical equilibrium is then restored, so far as this: the tree
yields up again the moisture it had drawn from the earth and the air,
though it does not return it each to each; for the vapor carried off by
transpiration greatly exceeds the quantity of water absorbed by the
foliage from the atmosphere, and the amount, if any, carried back to the
ground by the roots.
The present estimates of some eminent vegetable physiologists in regard
to the quantity of aqueous vapor exhaled by trees and taken up by the
atmosphere are much greater than those of former inquirers. Direct and
satisfactory experiments on this point are wanting, and it is not easy
to imagine how they could be made on a sufficiently extensive and
comprehensive scale. Our conclusions must therefore be drawn from
observations on small plants, or separate branches of trees, and of
course are subject to much uncertainty. Nevertheless, Schleiden, arguing
from such analogies, comes to the surprising result, that a wood
evaporates ten times as much water as it receives from atmospheric
precipitation. [Footnote: Fur Baum und Wald, pp. 46, 47, notes. Pfaff,
too, experimenting on branches of a living oak, weighed immediately
after being cut from the tree, and again after an exposure to the air
for three minutes, and computing the superficial measure of all the
leaves of the tree, concludes that an oak-tree evaporates, during the
season of growth, eight and a half times the mean amount of rain-fall on
an area equal to that shaded by the tree.] In the Northern and Eastern
States of the Union, the mean precipitation during the period of forest
growth, that is from the swelling of the buds in the spring to the
ripening of the fruit, the hardening of the young shoots, and the full
perfection of the other annual products of the tree, exceeds on the
average twenty-four inches. Taking this estimate, the evaporation from
the forest would be equal to a precipitation of two hundred and forty
inches, or very nearly one hundred and fifty standard gallons to the
square foot of surface.
The first questions which suggest themselves upon this statement are:
what becomes of this immense quantity of water and from what source does
the tree derive it We are told in reply that it is absorbed from the air
by the humus and mineral soil of the wood, and supplied again to the
tree through its roots, by a circulation analogous to that observed in
Ward's air-tight cases. When we recall the effect produced on the soil
even of a thick wood by a rain-fall of one inch, we find it hard to
believe that two hundred and forty times that quantity, received by the
ground between early spring and autumn, would not keep it in a state of
perpetual saturation, and speedily convert the forest into a bog.
No such power of absorption of moisture by the earth from the
atmosphere, or anything approaching it, has ever been shown by
experiment, and all scientific observation contradicts the supposition.
Schubler found that in seventy-two hours thoroughly dried humus, which
is capable of taking up twice its own weight of water in the liquid
state, absorbed from the atmosphere only twelve per cent. of its weight
of humidity; garden-earth five and one-fifth per cent. and ordinary
cultivated soil two and one-third per cent. After seventy-two hours,
and, in most of his experiments with thirteen different earths, after
forty-eight hours, no further absorption took place. Wilhelm,
experimenting with air-dried field-earth, exposed to air in contact with
water and protected by a bell-glass, found that the absorption amounted
in seventy-two hours to two per cent. and a very small fraction, nearly
the whole of which was taken up in the first forty-eight hours. In other
experiments with carefully heat-dried field-soil, the absorption was
five per cent. in eighty-four hours, and when the water was first warmed
to secure the complete saturation of the air, air-dried garden-earth
absorbed five and one-tenth per cent. in seventy-two hours.
In nature, the conditions are never so favorable to the absorption of
vapor as in those experiments. The ground is more compact and of course
offers less surface to the air, and, especially in the wood, it is
already in a state approaching saturation. Hence, both these physicists
conclude that the quantity of aqueous vapor absorbed by the earth from
the air is so inconsiderable "that we can ascribe to it no important
influence on vegetation." [Footnote: Wilhelm, Der Boden und das Wasser,
pp. 14,20.] Besides this, trees often grow luxuriantly on narrow ridges,
on steep declivities, on partially decayed stumps many feet above the
ground, on walls of high buildings, and on rocks, in situations where
the earth within reach of their roots could not possibly contain the
tenth part of the water which, according to Schleiden and Pfaff, they
evaporate in a day. There are, too, forests of great extent on high
bluffs and well-drained table-lands, where there can exist, neither in
the subsoil nor in infiltration from neighboring regions, an adequate
source of supply for such consumption. It must be remembered, also, that
in the wood the leaves of the trees shade each other, and only the
highest stratum of foliage receives the full influence of heat and
light; and besides, the air in the forest is almost stagnant, while in
the experiments of Unger, Marshal, Vaillant, Pfaff and others, the
branches were freely exposed to light, sun, and atmospheric currents.
Such observations can authorize no conclusions respecting the
quantitative action of leaves of forest trees in normal conditions.
Further, allowing two hundred days for the period of forest vital
action, the wood must, according to Schleiden's position, exhale a
quantity of moisture equal to an inch and one-fifth of precipitation per
day, and it is hardly conceivable that so large a volume of aqueous
vapor, in addition to the supply from other sources, could be diffused
through the ambient atmosphere without manifesting its presence by
ordinary hygrometrical tests much more energetically than it has been
proved to do, and in fact, the observations recorded by Ebermayer show
that though the RELATIVE humidity of the atmosphere is considerably
greater in the cooler temperature of the wood, its ABSOLUTE humidity
does not sensibly differ from that of the air in open ground. [Footnote:
Ebermeyer, Die Physikalischen, Einwirkungen des Waldes, i., pp. 150 et
seqq. It may be well here to guard my readers against the common error
which supposes that a humid condition of the AIR is necessarily
indicated by the presence of fog or visible vapor. The air is rendered
humid by containing INVISIBLE vapor, and it becomes drier by the
condensation of such vapor into fog, composed of solid globules or of
hollow vesicles of water--for it is a disputed point whether the
particles of fog are solid or vesicular. Hence, though the ambient
atmosphere may hold in suspension, in the form of fog, water enough to
obscure its transparency, and to produce the sensation of moisture on
the skin, the air, in which the finely divided water floats, may be
charged with even less than an average proportion of humidity.]
The daily discharge of a quantity of aqueous vapor corresponding to a
rain-fall of one inch and a fifth into the cool air of the forest would
produce a perpetual shower, or at least drizzle, unless, indeed, we
suppose a rapidity of absorption and condensation by the ground, and of
transmission through the soil to the roots and through them and the
vessels of the tree to the leaves, much greater than has been shown by
direct observation. Notwithstanding the high authority of Schleiden,
therefore, it seems impossible to reconcile his estimates with facts
commonly observed and well established by competent investigators. Hence
the important question of the supply, demand, and expenditure of water
by forest vegetation must remain undecided, until it can be determined
by something approaching to satisfactory direct experiment. [Footnote:
According to Cezanne, Surrell, Etude sur les Torrents, 2e edition, ii.,
p. 100, experiments reported in the Revue des Eaux et Forets for August,
1868, showed the evaporation from a living tree to be "almost
insignificant." Details are not given.]
Balance of Conflicting Influences of Forest on Atmospheric Heat and
Humidity.
We have shown that the forest, considered as dead matter, tends to
diminish the moisture of the air, by preventing the sun's rays from
reaching the ground and evaporating the water that falls upon the
surface, and also by spreading over the earth a spongy mantle which
sucks up and retains the humidity it receives from the atmosphere,
while, at the same time, this covering acts in the contrary direction by
accumulating, in a reservoir not wholly inaccessible to vaporizing
influences, the water of precipitation which might otherwise suddenly
sink deep into the bowels of the earth, or flow by superficial channels
to other climatic regions. We now see that, as a living organism, it
tends, on the one hand, to diminish the humidity of the air by sometimes
absorbing moisture from it, and, on the other, to increase that humidity
by pouring out into the atmosphere, in a vaporous form, the water it
draws up through its roots. This last operation, at the same time,
lowers the temperature of the air in contact with or proximity to the
wood, by the same law as in other cases of the conversion of water into
vapor.
As I have repeatedly said, we cannot measure the value of any one of
those elements of climatic disturbance, raising or lowering of
temperature, increase or diminution of humidity, nor can we say that in
any one season, any one year, or any one fixed cycle, however long or
short, they balance and compensate each other. They are sometimes, but
certainly not always, contemporaneous in their action, whether their
tendency is in the same or in opposite directions, and, therefore, their
influence is sometimes cumulative, sometimes conflicting; but, upon the
whole, their general effect is to mitigate extremes of atmospheric heat
and cold, moisture and drought. They serve as equalizers of temperature
and humidity, and it is highly probable that, in analogy with most other
works and workings of nature, they, at certain or uncertain periods,
restore the equilibrium which, whether as lifeless masses or as living
organisms, they may have temporarily disturbed. [Footnote: There is one
fact which I have nowhere seen noticed, but which seems to me to have an
important bearing on the question whether forests tend to maintain an
equilibrium between the various causes of hygroscopic action, and
consequently to keep the air within their precincts in an approximately
constant condition, so far as this meteorological element is concerned.
I refer to the absence of fog or visible vapor in thick woods in full
leaf, even when the air of the neighboring open grounds is so heavily
charged with condensed vapor as completely to obscure the sun. The
temperature of the atmosphere in the forest is not subject to so sudden
and extreme variations as that of cleared ground, but at the same time
it is far from constant, and so large a supply of vapor as is poured out
by the foliage of the trees could not fail to be sometimes condensed
into fog by the same causes as in the case of the adjacent meadows,
which are often covered with a dense mist while the forest-air remains
clear, were there not some potent counteracting influence always in
action. This influence, I believe, is to be found partly in the
equalization of the temperature of the forest, and partly in the balance
between the humidity exhaled by the trees and that absorbed and
condensed invisibly by the earth.] When, therefore, man destroys these
natural harmonizors of climatic discords, he sacrifices an important
conservative power, though it is far from certain that he has thereby
affected the mean, however much he may have exaggerated the extremes of
atmospheric temperature and humidity, or, in other words, may have
increased the range and lengthened the scale of thermometric and
hygrometric variation.
Special Influence of Woods on Precipitation.
With the question of the action of forests upon temperature and upon
atmospheric humidity is intimately connected that of their influence
upon precipitation, which they may affect by increasing or diminishing
the warmth of the air and by absorbing or exhaling uncombincd gas and
aqueous vapor. The forest being a natural arrangement, the presumption
is that it exercises a conservative action, or at least a compensating
one, and consequently that its destruction must tend to produce
pluviometrical disturbances as well as thermometrical variations. And
this is the opinion of perhaps the greatest number of observers. Indeed,
it is almost impossible to suppose that, under certain conditions of
time and place, the quantity and the periods of rain should not depend,
more or less, upon the presence or absence of forests; and without
insisting that the removal of the forest has diminished the sum-total
of snow and rain, we may well admit that it has lessened the quantity
which annually falls within particular limits. Various theoretical
considerations make this probable, the most obvious argument, perhaps,
being that drawn from the generally admitted fact, that the summer and
even the mean temperature of the forest is below that of the open
country in the same latitude. If the air in a wood is cooler than that
around it, it must reduce the temperature of the atmospheric stratum
immediately above it, and, of course, whenever a saturated current
sweeps over it, it must produce precipitation which would fall upon it,
or at a greater or less distance from it.
We must here take into the account a very important consideration. It is
not universally or even generally true, that the atmosphere returns its
condensed humidity to the local source from which it receives it. The
air is constantly in motion,
--howling tempests scour amain
From sea to land, from land to sea;
[Footnote: Und Sturme brausen um die Wette
Vom Meer aufs Land, vom Land aufs Meer.
Goethe, Faust, Song of the Archangels.]
and, therefore, it is always probable that the evaporation drawn up by
the atmosphere from a given river, or sea, or forest, or meadow, will be
discharged by precipitation, not at or near the point where it rose, but
at a distance of miles, leagues, or even degrees. The currents of the
upper air are invisible, and they leave behind them no landmark to
record their track. We know not whence they come, or whither they go. We
have a certain rapidly increasing acquaintance with the laws of general
atmospheric motion, but of the origin and limits, the beginning and end
of that motion, as it manifests itself at any particular time and place,
we know nothing. We cannot say where or when the vapor, exhaled to-day
from the lake on which we float, will be condensed and fall; whether it
will waste itself on a barren desert, refresh upland pastures, descend
in snow on Alpine heights, or contribute to swell a distant torrent
which shall lay waste square miles of fertile corn-land; nor do we know
whether the rain which feeds our brooklets is due to the transpiration
from a neighboring forest, or to the evaporation from a far-off sea. If,
therefore, it were proved that the annual quantity of rain and dew is
now as great on the plains of Castile, for example, as it was when they
were covered with the native forest, it would by no means follow that
those woods did not augment the amount of precipitation elsewhere. The
whole problem of the pluviometrical influence of the forest, general or
local, is so exceedingly complex and difficult that it cannot, with our
present means of knowledge, be decided upon a priori grounds. It must
now be regarded as a question of fact which would probably admit of
scientific explanation if it were once established what the actual fact
is.
Unfortunately, the evidence is conflicting in tendency, and sometimes
equivocal in interpretation, but I believe that a majority of the
foresters and physicists who have studied the question are of opinion
that in many, if not in all cases, the destruction of the woods has been
followed by a diminution in the annual quantity of rain and dew. Indeed,
it has long been a popularly settled belief that vegetation and the
condensation and fall of atmospheric moisture are reciprocally necessary
to each other, and even the poets sing of
Afric's barren sand,
Where nought can grow, because it raineth not,
And where no rain can fall to bless the land,
Because nought grows there.
[Footnote: Det golde Strog i Afrika,
Der Intet voxe kan, da ei det regner,
Og, omvendt, ingen Regn kan falde, da
Der Intet voxer.
Paudan-Muller, Adam Hamo, ii., 408.]
Before going further with the discussion, however, it is well to remark
that the comparative rarity or frequency of inundations in earlier or
later centuries is not necessarily, in most cases not probably, entitled
to any weight whatever, as a proof that more or less rain fell formerly
than now; because the accumulation of water in the channel of a river
depends far less upon the quantity of precipitation in its valley, than
upon the rapidity with which it is conducted, on or under the surface of
the ground, to the central artery that drains the basin. But this point
will be more fully discussed in a subsequent chapter.
In writers on the subject we are discussing, we find many positive
assertions about the diminution of rain in countries which have been
stripped of wood within the historic period, but these assertions very
rarely rest upon any other proof than the doubtful recollection of
unscientific observers, and I am unable to refer to a single instance
where the records of the rain-gauge, for a considerable period before
and after the felling or planting of extensive woods, can be appealed to
in support of either side of the question. The scientific reputation of
many writers who have maintained that precipitation has been diminished
in particular localities by the destruction of forests, or augmented by
planting them, has led the public to suppose that their assertions
rested on sufficient proof. We cannot affirm that in none of these cases
did such proof exist, but I am not aware that it has ever been produced.
[Footnote: Among recent writers, Clave, Schacht, Sir John F. W.
Herschel, Hohenstein, Barth, Asbjornsen, Boussingault, and others,
maintain that forests tend to produce rain and clearings to diminish it,
and they refer to numerous facts of observation in support of this
doctrine; but in none of these does it appear that these observations
are supported by actual pluviometrical measure. So far as I know, the
earliest expression of the opinion that forests promote precipitation is
that attributed to Christopher Columbus, in the Historie del S. D.
Fernando Colombo, Venetia, 157l, cap. lviii., where it is said that the
Admiral ascribed the daily showers which fell in the West Indies about
vespers to "the great forests and trees of those countries," and
remarked that the same effect was formerly produced by the same cause in
the Canary and Madeira Islands and in the Azores, but that "now that the
many woods and trees that covered them have been felled, there are not
produced so many clouds and rains as before."
Mr. H. Harrisse, in his very learned and able critical essay, Fernand
Colomb, sa Vie et ses Oeuvres, Paris, 1872, has made it at least
extremely probable that the Historie is a spurious work. The compiler
may have found this observation in some of the writings of Columbus now
lost, but however that may be, the fact, which Humboldt mentions in
Cosmos with much interest, still remains, that the doctrine in question
was held, if not by the great discoverer himself, at least by one of his
pretended biographers, as early as the year 1571.]
The effect of the forest on precipitation, then, is by no means free
from doubt, and we cannot positively affirm that the total annual
quantity of rain is even locally diminished or increased by the
destruction of the woods, though both theoretical considerations and the
balance of testimony strongly favor the opinion that more rain falls in
wooded than in open countries. One important conclusion, at least, upon
the meteorological influence of forests is certain and undisputed: the
proposition, namely, that, within their own limits, and near their own
borders, they maintain a more uniform degree of humidity in the
atmosphere than is observed in cleared grounds. Scarcely less can it be
questioned that they tend to promote the frequency of showers, and, if
they do not augment the amount of precipitation, they probably equalize
its distribution through the different seasons. [Footnote: The strongest
direct evidence which I am able to refer to in support of the
proposition that the woods produce even a local augmentation of
precipitation is furnished by the observations of Mathieu, sub-director
of the Forest-School at Nancy. His pluviometrical measurements,
continued for three years, 1866-1868, show that during that period the
annual mean of rain-fall in the centre of the wooded district of
Cinq-Tranchees, at Belle Fontaine on the borders of the forest, and at
Amance, in an open cultivated territory in the same vicinity, was
respectively as the numbers 1,000, 957, and 853.
The alleged augmentation of rain-fall in Lower Egypt, in consequence of
large plantations by Mehemet Ali, is very frequently appealed to as a
proof of this influence of the forest, and this case has become a
regular common-place in all discussions of the question. It is, however,
open to the same objection as the alleged instances of the diminution of
precipitation in consequence of the felling of the forest.